Various wireless access technologies have been proposed or implemented to enable mobile stations to perform communications with other mobile stations or with wired terminals coupled to wired networks. Examples of wireless access technologies include GSM (Global System for Mobile communications) or UMTS (Universal Mobile Telecommunications System) technologies, defined by the Third Generation Partnership Project (3GPP); CDMA 2000 (Code Division Multiple Access 2000) technologies, defined by 3GPP2; or other wireless access technologies.
As part of the continuing evolution of wireless access technologies to improve spectral efficiency, to improve services, to lower costs, and so forth, new standards have been proposed. One such new standard is the Long Term Evolution (LTE) standard from 3GPP, which seeks to enhance the UMTS wireless network.
Another type of wireless access technology is the WiMax (Worldwide Interoperability for Microwave Access) technology. WiMax is based on the IEEE (Institute of Electrical and Electronics Engineers) 802.16 Standard. The WiMax wireless access technology is designed to provide wireless broadband access.
In some wireless access technologies, orthogonal frequency division multiple access (OFDMA) is employed for wireless communications between a base station and a mobile station. OFDMA enables multiple access by multiple users using the orthogonal frequency division multiplexing (OFDM) technology. OFDM defines a relatively large number of closely-spaced orthogonal sub-carriers for carrying data. Data can be divided into several parallel data streams that are communicated over respective sub-carriers. With OFDM, the frequencies of the sub-carriers are selected such that the sub-carriers are orthogonal to each other, which eliminates or reduces cross-talk between the sub-carriers. With OFDMA, different OFDM sub-carriers are assigned to different users, such that data for multiple users can be sent in parallel between the base station and mobile station.
OFDMA used in the uplink (from mobile station to base station) provides various benefits, such as superior diversity gain, more convenient and flexible multi-user multiplexing, reduced complexity at the mobile station since no additional digital Fourier transform (DFT) processing is required, and superior MIMO (multiple input multiple output) decoding performance. MIMO refers to wireless transmission in which the transmitter has multiple antennas (multiple output) and the receiver also has multiple antennas (multiple input). The multiple antennas at the transmitter and receiver can correspond to the multiple sub-carriers used with OFDM.
However, use of OFDMA in the uplink can result in more expensive power amplifiers in transmitters of mobile stations due to larger power amplifier backoff requirements. Power amplifier backoff refers to operating the power amplifier at an output power level that is lower than the peak power level. A larger power amplifier backoff (lower average power level relative to the peak power level) reduces the efficiency of the power amplifier. Another issue associated with use of OFDMA in the uplink may be reduced link budget, which represents the expected received power at a receiver due to the transmitted power, antenna gains, and any losses in the medium, such as propagation losses and other losses. Reduced link budget means a lower expected received power at a receiver, such that the coverage that can be provided for mobile stations within a cell is reduced.
Thus, the limited transmit power used with OFDMA causes cell coverage and throughput to be limited.